The invention relates to a plasma picture screen provided with a phosphor layer which comprises a red, Eu3+-activated phosphor.
Plasma picture screens are capable of showing color images of high resolution, large screen diagonal, and are of a compact construction. A plasma picture screen comprises a hermetically closed glass cell which is filled with a gas and which comprises electrodes arranged in a grid. The application of a voltage gives rise to a gas discharge which generates light in the ultraviolet range. This light is converted into visible light by phosphors and is emitted to the viewer through the front plate of the glass cell.
A red phosphor which is used in many plasma picture screens is (Y,Gd)BO3:Eu, because this phosphor shows a higher light output when excited by VUV radiation than other red-emitting phosphors. Considerable disadvantages of this phosphor are on the one hand the color point, which is too close to orange for video applications with x=0.64 and y=0.35, and on the other hand the comparatively long decay time of xcfx841/10=9 ms.
The orange color point of (Y,Gd)BO3:Eu leads to a smaller range of colors in plasma picture screens compared with cathode ray tubes in which Y2O2S:Eu is used as the red phosphor. The latter has a color point of x=0.66 and y=0.33.
The color point and the decay time xcfx841/10 of a phosphor activated by Eu3+ depend on the local symmetry of the lattice locations occupied by the Eu3+ cations in the phosphor host lattice. In YBO3, GdBO3, and (Y,Gd)BO3, which have a pseudo-vaterite lattice, the Eu3+ cation occupies two crystallographic locations with a slightly distorted S6 symmetry. As a consequence, electrical dipole transitions 5D0-7F2,4, which are forbidden in an S6 symmetry or D3d symmetry, can be observed.
The spectroscopic selection rules are less stringent owing to the distortion, and the emission spectrum of (Y,Gd)BO3:Eu shows two intensive lines at 612 and 627 nm which can be ascribed to the 5D0-7F2 transition. Two emission lines are obtained owing to the crystal field splitting of the 7F2 level. The most intensive emission line, however, is formed by the magnetic dipole transition 5D0-7F1 at 595 nm. The high intensity of the emission line at 595 nm is responsible for the orange color point of this phosphor. The smaller transition probability of a magnetic dipole transition leads to longer emission decay times of this phosphor.
The invention has for its object to provide a red phosphor with an improved color point and a shorter decay time xcfx841/10 for plasma picture screens.
The object is achieved by means of a plasma picture screen provided with a phosphor layer which comprises a red, Eu3+-activated phosphor, which screen has a host lattice with a crystal structure in which the Eu3+ cations occupy locations without inversion symmetry.
Electrical dipole transitions 5D0-7F2,4,6 of the Eu3+ cation are allowed according to the spectroscopic selection rules in a phosphor in which the Eu3+ cation occupies lattice locations in the crystal structure of the host lattice which have no inversion symmetry center. Such transitions are forbidden when the Eu3+ cation occupies locations in the lattice which have an inversion symmetry. In that case only a magnetic dipole transition 5D0-7F1 is allowed.
Accordingly, phosphors having a lattice in which the Eu3+ cations occupy locations without inversion symmetry show intensive emission bands in a range from 610 to 630 nm which are based on electrical dipole transitions.
It is preferable that the phosphor comprises a lattice with a fergusonite structure, aragonite structure, thortveitite structure, matlockite structure, or xenotim structure.
The Eu3+ cation occupies a trivalent lattice location without inversion symmetry in these host lattices.
It is furthermore preferable that the phosphor is chosen from the group comprising Ba2Gd2(SixGe1-x)4O13, (YxGd1-x)NbO4, (YxGd1-x)TaO4, (YxGd1-x)OF, (YxGd1-x)OCl and (ScxLu1-x)2Si2O7, for which it holds in all cases that 0xe2x89xa6xxe2x89xa61.
These phosphors with Eu3+ as the activator have color points with xxe2x89xa70.65 and yxe2x89xa60.35.
It is furthermore preferred that the proportion of Eu3+ in the phosphor lies between 0.5 and 10 mole %.
The proportion of Eu3+ must not be too low, because then the absorption of the UV radiation is too low. On the other hand, the proportion must not be too high, because then the light output is too small owing to concentration quenching.